From in the past, there have been many studies of methods of refining the structure of a cold-rolled steel sheet in order to improve the mechanical properties of the steel sheet.
These methods can be generally divided into the following categories (1)-(3).
(1) A first method is a method in which a large amount of elements which suppress grain growth such as Ti, Nb, and Mo are added to refine austenite grains which are formed at the time of annealing after cold rolling, thereby refining ferrite grains which are formed by transformation from austenite in subsequent cooling.
(2) A second method is a method in which heating to a single-phase austenitic region in the above-described annealing is carried out by rapid heating followed by holding for an extremely short length of time to prevent coarsening of the structure.
(3) A third method is a method in which cold rolling and annealing are carried out on a hot-rolled steel sheet manufactured by rapid cooling immediately after hot rolling. Below, this method of manufacturing a hot-rolled steel sheet will be referred to as the immediate cooling method.
With respect to the above-described first method, Patent Document 1, for example, discloses a cold-rolled steel sheet having a steel structure primarily comprising ferrite with an average grain diameter of at most 3.5 μm. Patent Document 2 discloses a cold-rolled steel sheet having a structure comprising ferrite and a low temperature transformation phase constituted by one or more selected from martensite, bainite, and retained γ (retained austenite). The average grain diameter of the low temperature transformation phase is at most 2 μm and its volume fraction is 10-50%.
Concerning the second method, Patent Document 3, for example, discloses a method in which a hot-rolled steel sheet which was coiled at 500° C. or above is cold-rolled and then annealed by rapid heating at a rate of at least 30° C. per second in the temperature range from room temperature to 750° C. and limiting the holding time at an annealing temperature in the range of 750-900° C., thereby causing transformation from unrecrystallized ferrite to fine austenite, from which fine ferrite is formed at the time of cooling. Patent Document 4 describes a method of manufacturing a bake hardenable high strength cold-rolled steel sheet comprising cold rolling a hot-rolled steel sheet obtained by usual hot rolling, and then subjecting the steel sheet to continuous annealing by heating to a temperature range of 730-830° C. at a rate of 300-2000° C. per second in a temperature region of at least 500° C. followed by holding in the temperature range for at most 2 seconds.
Concerning the third method, Patent Document 5 discloses a method in which cold rolling is carried out on a hot-rolled steel sheet produced by the immediate rapid cooling method in which cooling is started a short time after hot rolling. For example, a hot-rolled steel sheet having a fine structure and predominantly comprising ferrite with a small average grain diameter is produced by cooling to a temperature of 720° C. or below at a cooling rate of at least 400° C. per second within 0.4 seconds after the completion of hot rolling and is used as a starting material for cold rolling, and cold rolling and annealing are carried out in a usual manner.
In Patent Document 5, a region which is surrounded by a high angle grain boundary for which the misorientation (also referred to as the tilt angle) is at least 15° is defined as a single (crystal) grain. Accordingly, a hot-rolled steel sheet having a fine structure disclosed by Patent Document 5 is characterized by having a large number of high angle grain boundaries.